An electronic drive control for a contactor operating mechanism is described in German Publication DE 299 09 901 U1. The drive control essentially includes a rectifier circuit supplied via control inputs, a series circuit which is composed of the operating coil and a pulse-width controlled transistor switch and is supplied by the rectifier circuit, two voltage divider circuits which scan the output of the rectifier circuit and are isolated on the input side by an isolation diode, as well as an electronic circuit including a microprocessor and two memories. Control signals for the pickup and holding modes of the operating coil are supplied to the transistor by the electronic circuit; the corresponding pulse widths in the pickup and holding modes being determined via the associated memory in accordance with the output signal of the associated voltage divider. Moreover, it is known from German Publication DE 299 09 904 U1 to provide such electronic drive controls with a first transistor switch for controlling the pickup current and a second transistor switch for controlling the holding current. Such electronic drive controls have the disadvantage of having a high degree of complexity, which is due to the electronic circuit and is of particular consequence for operating mechanisms of lower-rated electromagnetic switching devices.
German Publication DE 92 16 041 U1 describes a circuit arrangement for controlling a relay. The series circuit of the operating coil and the first transistor switch is connected to a DC operating voltage, and the series circuit of a holding resistor and a second transistor switch is placed in parallel with the switching path of the first transistor switch. A d.c. control input is connected, via a differentiating timer including a capacitor and a discharge resistor, to the control electrode of the first transistor switch and, via a series resistor, to the control electrode of the second transistor switch. After a control voltage has been applied, both the first and second transistor switches are turned on, as a result of which a pickup voltage is applied across the operating coil; the pickup voltage obtained being the DC operating voltage minus the saturation voltage of the first transistor switch. When the capacitor voltage of the differentiating element has dropped, the first transistor goes to the OFF state. Consequently, the operating coil is then only supplied with a holding current, which is essentially obtained from the ratio of the DC operating voltage to the sum of the holding resistance and the ohmic resistance of the operating coil. After the control voltage has been removed, the second transistor switch is also turned off, thereby switching off the relay. In the case of this control circuit, both the pickup response and the reliability and heat losses in the holding mode are highly dependent on changes and fluctuations in the DC operating voltage. The drive control, which is only suitable for DC voltage operation, uses a control voltage in addition to the operating voltage; the control voltage being independent of the operating voltage. An additional significant amount of power is lost through the holding resistor.
German Patent DE 44 10 819 C2 discloses a circuit arrangement which is intended to operate a relay and which, in turn, has a first transistor switch, which is turned on during the pickup phase, and a second transistor switch, which is placed in series with the operating coil and a holding resistor and connected to an operation voltage and which is turned on when the relay is in the ON state. The switching path of the first transistor switch is placed in parallel with the holding resistor. A d.c. control input is connected via a voltage divider to the control electrode of the second transistor switch. The control electrode of the first transistor switch is connected to the junction point of the first transistor switch, the second transistor switch and the holding resistor via an integrating timer including a charging resistor and a capacitor. When the relay is in the OFF state, the capacitor is charged via the operating coil, the holding resistor and the charging resistor so that both transistor switches are turned on when a control voltage is applied. In this connection, the pickup voltage obtained for the operating coil equals the operating voltage minus the sum of the saturation voltages of the two transistor switches. At the same time, the capacitor begins to discharge through the series resistor and the switching path of the second transistor switch. After the capacitor voltage has fallen below a threshold value, the first transistor is turned off. Consequently, the operating coil is then only supplied with a holding current, which is essentially obtained from the ratio of the DC operating voltage to the sum of the holding resistance and the ohmic resistance of the operating coil. After the control voltage has been removed, the second transistor switch is also turned off, thereby switching off the relay. This drive control presents the above-described disadvantages of the approach of German Publication DE 92 16 041 U1 and requires an operating voltage to be provided continuously or at least with sufficient time before the relay is switched on.
German Patent 196 38 260 C2 discloses a circuit arrangement for controlling small solenoid coils, including a transistor switch connected in series with the solenoid coil. Upon application of a control voltage, the turned-on transistor switch applies a high pickup current to the solenoid coil during a time period set by a differentiating timer. After that, the holding current is determined by a series circuit which is composed of a holding resistor and a light-emitting diode and is placed in parallel with the switching path of the transistor switch. Here too, the pickup and holding currents are highly dependent on the magnitude of the control voltage, and a significant amount of power is lost through the holding resistor.